Just be careful that the radiation at 120MHz from the 70cms antenna - there will be some - doesn't give a problem for aircraft. Also, any strong FM BC stations in 94 - 96 MHz can mix with the 4th harmonic and come out in 2m.

Depends on the 70cm antenna performance at 120 MHz - and its impedance at 120MHz and the leakage inductance of the tuned circuit etc. In Europe, all the receiver radiation is required to be -57dBm conducted to the antenna for frequencies below 1GHz. That probably means getting some 60dB of attenuation...

More of a problem is the FM broadcast breakthrough if there are strong local signals in 94 - 96 MHz - also breakthrough from DAB operating around 216 MHz. Adding a quarter wave short circuit coax stub at 70cms across the input would help enormously - about 12cms would do it with solid polythene dielectric coax. It would help with radiation, too.

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